The most effective method for removing carbon dioxide (CO2) from gas mixtures containing carbon dioxide such as gas mixtures obtained from hydrogen, steel and cement production processes, combustion exhaust gas generated from power plants using fossil fuels, and natural gas includes a physical absorption method using an organic solvent when the concentration of carbon dioxide is high, and a chemical absorption method using an amine-based aqueous solution when the concentration of carbon dioxide is low. Among them, the physical absorption method suitable for a gas mixture having a high concentration of carbon dioxide has already proved its economic efficiency and effectiveness in various commercialization processes such as hydrogen production and purification of natural gas, etc. On the other hand, when carbon dioxide is removed from combustion gas or the like having a low concentration of carbon dioxide, although chemical absorbents are much more effective than physical absorbents, the commercialization of the chemical absorbents remains unsolved due to the problem of high absorbent regeneration energy.
Amine aqueous solutions such as monoethanolamine (MEA), diethanolamine (DEA), piperazine and the like have been most extensively studied as the chemical absorbents, because these amine-based absorbents react with carbon dioxide to readily form stable carbamate compounds, and also, these compounds can be decomposed again into carbon dioxide and amines by heat. However, carbon dioxide capturing processes using these amine-based absorbents impose some serious problems. In particular, as the decomposition temperature is as high as 120° C. or more due to high thermal-chemical stability of carbamates produced from the reaction with carbon dioxide, in addition to a problem of excessive regeneration energy consumption (for MEA, 4.0˜4.2 GJ of regeneration energy is needed per ton of carbon dioxide), a problem associated with excessive volatilization loss of amines (4 Kg/ton of MEA) due to the high regeneration temperature, and a problem of supplementing the absorbents accompanied by this, have been pointed out as disadvantages.
In order to overcome the disadvantages of these amine-based aqueous solution absorbents, attempts have been made to use an alkanolamine having a steric hindrance around the amine group of the alkanolamine as absorbent, and typical examples thereof include 2-amino-2-methyl-1-propanol (AMP) which is a primary amine. The AMP is characterized by forming a bicarbonate compound ([AMPH][HCO3]) which can be readily regenerated compared to a carbamate during the reaction with carbon dioxide, and thus has an advantage of having regeneration energy which is 30% lower than that of MEA, but it also has a disadvantage in that it does not even reach 50% of the carbon dioxide absorption rate of MEA.
As a means to increase the absorption rate of AMP, Mitsubishi Heavy Industries and Kansai Electric Power Co., Inc. have jointly developed a novel absorbent in which piperazine as a secondary cyclic amine is added to AMP (Japanese Patent No. 3197173). However, the absorbent disclosed in this patent has a problem in that precipitation occurs during a carbon dioxide absorption process, and further, piperazine and carbon dioxide react with each other to form a thermally more stable carbamate in addition to the bicarbonate compound, and thus, there is a problem that regeneration is difficult.
In addition, there is also known a method which can reduce regeneration energy by using an alkali carbonate such as sodium carbonate or potassium carbonate as a carbon dioxide absorbent instead of a primary alkanolamine absorbent such as MEA, but it has a disadvantage of a low carbon dioxide absorption rate. As one of the methods for increasing carbon dioxide absorption rate, it has been reported in Published International Application WO 2004-089512 A1 that, when piperazine or a derivative thereof is added to potassium carbonate, the carbon dioxide absorption rate is greatly increased, but the problem of precipitation formation resulting from the use of carbonate still remains a challenge to be solved.
When carbon dioxide is captured using an amine-based absorbent using water as solvent, about 70% or more of consumed energy is used for regenerating the absorbent, and among them, 50% or more of the energy is known as energy required to vaporize water, that is, the energy attributed to the latent heat of water. This implies that, when the absorbent solvent is replaced by an organic solvent having a boiling point higher than the regeneration temperature of the absorbent and a low specific heat, instead of water having a high latent heat and specific heat, the energy consumed in the capturing process may be significantly reduced. Based on these findings, Published International Applications WO2012-034921 A1 and WO2012-093853 A1 disclose that a solution dissolved in AMP or t-butylaminoethanol (TBAE) having a steric hinderance can be used as a carbon dioxide absorbent. However, the absorbents disclosed in these patents have a severe disadvantage of heavy alcohol loss in the absorption and regeneration processes, because the absorption rate of carbon dioxide is significantly lower than that of MEA, and it can only be effective in alcohols having a low boiling point such as methanol. Further, the absorbents show the same performance as in methanol even in the presence of ethylene glycol (EG) solvent which is an alcohol having a high boiling point, but there exists a disadvantage in that the viscosity of the solution is excessively increased after the absorption of carbon dioxide, thereby imposing a restriction on the circulation of absorbent.
In addition, as a means to overcome the disadvantages of the conventional absorbents, as suggested in U.S. Pat. Nos. 6,849,774, 6,623,659, and U.S. Patent Application Publication No. 2008/0146849, attempts have been made to use ionic liquids which maintain a liquid phase at a low temperature of 100° C. or less while having no volatility and high thermal stability as non-aqueous absorbents. However, in order to synthesize these ionic liquids, it is necessary to go through a complicated production process involving two or more steps, and not only the production cost is excessively high, but also the viscosity of the absorption solution is high. Further, the carbon dioxide absorptivity and absorption rate at a low pressure are excessively low, and thus, it is not suitable for capturing carbon dioxide from exhaust gas emitted to the atmosphere after combustion.